I just finished a simple benchmark of the various Git ruby libraries out there, my git gem, Err's Grit (as used by GitHub, and my new pure ruby Git lib, Git-Ruby. Turns out that because the other two libraries are running system calls for everything, the pure ruby version is pretty dang fast:

Git Gem:
             user     system      total        real
objects   0.030000   0.130000   0.730000 (  0.759323)
log       0.480000   0.420000   2.210000 (  2.258663)
branch    0.120000   0.920000   5.450000 (  5.434611)

Grit :
             user     system      total        real
objects   0.020000   0.120000   0.590000 (  0.587605)
log       0.280000   0.430000   1.990000 (  1.958004)
branch    0.020000   0.160000   0.850000 (  0.877369)

Git-Ruby (pure ruby) :
             user     system      total        real
objects   0.030000   0.010000   0.040000 (  0.049364)
log       0.950000   0.100000   1.050000 (  1.059527)
branch    0.130000   0.070000   0.200000 (  0.191696)

Sweet. Still a lot of work to do, though.